Calculate Accuracy of SCXI 1104C with PXI 6052E

[sjarago]

I am unable to reproduce the results obtained from the NI "Accuracy Calculator" with the configuration listed above.  For an input of 41.8 V, the "Accuracy Calculator" returns:

DAQ Absolute Accuracy = 8.1626 mV

SCXI Absolute Accuracy = 15.7300 mV

Total System Accuracy = 17.7218 mV (This is just RSS of first two numbers)

The numbers I am getting for an accuracy are not even close, no matter how I look at them.  Please help.  Thanks.

I am referencing:

SCXI 1104/C User Manual, Feb 2000 Edition, Part# 322147B-01

NI 6052E Family Specifications, 370723C-01, Dec 05.

[Matt_A]

Hello sjarago,

There are some important things to keep in mind when using the Accuracy Calculator. I recreated the values you gave from the accuracy calculator with the following parameters:

Parameters

Signal Range for DAQ device   -5 to 5 V

Operating temperature   25 C

Polarity   Bipolar

Number of Readings   Average of 100 readings

Time from last calibration of DAQ device   1 year

Specifically, I would direct you to the “Number of Readings” and "Time from last calibration of DAQ device”. If either of these parameters is different than your setup, you may not receive the same accuracy.

Additionally, I wanted to ask you what you meant when you said the numbers you were getting for accuracy were not the same. Are you saying that you are physically measuring a channel and the values you are seeing are different? Or, have you calculated different numbers using the documents you referenced?

For more specifics on how the Accuracy Calculator determines its results, I would direct you to the formulas listed here. This page should help clarify some of the confusion regarding the numbers listed.

I hope this information is helpful. Please let me know if you have any additional questions.

[sjarago]

Thanks for replying.  Sorry my question was not clear.  From the documents I have referenced, I have calculated different numbers than those returned to me by the Accuracy Calculator.  Specifically, I am unable to come up with an SCXI Absolute Accuracy of 15.73 mV for a 41.8 V input based on the specifications listed in Appendix A of the SCXI manual.

I have a follow on question, which is, what is the voltage that is output from the 1104C for that (or any) input to the DAQ?  Obviously the 1104C is scaling down the 41.8 V signal to one that can be read on the -5 to 5 V range on the DAQ.  Is this a linear scaling?  I cannot compute the DAQ Absolute Accuracy without this number.

Thanks for your help,

Steven

[Matt_A]

Steven,

What formula are you using to determine your values? What values are you getting? I am not sure how you are determining your values, so I do not feel confident commenting on your methods.

In regards to your second question, page B-1 of the SCXI-1104C user manual (page 48 of the pdf) gives the following information:

What are the similarities and differences between the SCXI-1102/B/C
and the SCXI-1104/C?
These modules have a similar internal architecture. Each channel has its
own amplifier and filter, which makes scanning trouble-free compared to a
shared-amplifier-and-filter architecture. All 32 differential input channels
are multiplexed to a single output channel and then routed to the DAQ
device, either directly or through the SCXIbus. Both modules have a fixed
cutoff frequency.
You can configure the SCXI-1102/B/C for a gain of either 1 or 100 while
the SCXI-1104/C has a fixed gain of 0.1 only.

[sjarago]

Matt,

I understand if you are not confident commenting on my methods because there are too many things to factor in.  Instead, please do the following:  Show me, in painstaking detail, how to determine the accuracies of my instruments for an input of 41.8 V.  Specifically, show me using the conditions listed in the first and second part of this thread.  Please reference the documents, version numbers, pages, etc.

Honestly, I don’t care if the number you come up with matches the Accuracy calculator or not.  I just need to be able to properly document how I determined my accuracies.

Thanks for your help,

Steven